The size-mass and Faber–Jackson relationships. Left panel: stellar mass vs. radius relations for the Illustris-1 (open red squares) and the Illustris-TNG-100 (blue dots) samples at z = 0 and comparison of the models with the WINGS data (black dots). There are 2400 objects for the Illustris-1 sample and about 600 objects for the Illustris-TNG-100. The mean radii of Illustris-1 are smaller by about a factor of two for stellar masses lower than about 6 × 10¹⁰ M_⊙, while they are nearly equal (if not slightly larger) above this limit. Right panel: L − σ plane of the same data. The symbols and color-coding are the same as in the left panel.